This research project is designed to advance diagnostic and therapeutic applications of ultrasound by increasing our understanding of the mechanisms of interaction of ultrasound with tissues at the molecular level. The long term objective of this program is to identify mechanisms of absorption of ultrasound in biological material at the molecular level by studying the dependence of absorption and propagation speed upon various aspects of the physical and chemical state of biomolecules in solution and suspension. During the proposed project period, the absorption and propagation speed of several lipids in liquid, liquid-crystalline, and crystalline states will be examined. The absorption and propagation speed characteristics will be related to the lipid physical and chemical states to determine important characteristics relevant to ultrasound interaction with lipids. Specific experiments include (1) investigate the role of lipid molecular structure through selection of various acyl chains and polar head groups, (2) investigate the effects of ionic strength and pH of the aqueous solvent on absorption of lipid suspensions, (3) investigate absorption in lipid systems of varying degrees of molecular aggregation and interaction through studies of liquid droplets, micelles, vesicles, and liposomes. This research plan is aimed at using lipids as a model for obtaining a better understanding of the mechanisms of the absorption of ultrasound in biological material. Lipid suspensions provide useful models for looking at several aspects of this problem.